Continuous separation system

ABSTRACT

A method of separating solids and liquid material in a slurry. The solids are continuously purged, washed, dried and removed with a minimum amount of mechanical handling. The slurry is continuously deposited on a moving horizontal rotary screen and condensible vapor is introduced above the layer while the pressure is reduced below the layer. Liquid is withdrawn from underneath the slurry layer with the vapor passing through the layer becoming superheated upon passage to evaporate further liquid from the solids. A stream of gas, which may be the same condensible vapor, is directed at the solids to entrain them and remove them from the screen.

This is a division of application Ser. No. 66,712, filed Aug. 15, 1979,now U.S. Pat. No. 4,256,582, which is itself a continuation in part ofSer. No. 906,078 filed May 15, 1978, now abandoned.

FIELD OF THE INVENTION

This invention relates generally to separation of solids and liquids inslurries, and more specifically to separation of sugar crystals frommolasses in massecuite.

BACKGROUND OF THE INVENTION

Most separation of commercial sugar crystals from molasses occurs incentrifugal separators. However, this technique has many disadvantagesin that, in order to develop the required centrifugal force, the basketson which the screens are mounted must be rotated at relatively highspeeds (usually from about 1,000 to 1,800 rpm). Operation thus requiresconsiderable power, typically in the neighborhood of 50 to 100horsepower, further necessitating careful dynamic balance of therotating parts and sturdy construction of the entire system. A furtherdifficulty with centrifugal separators is that they operate in a batchmode and thus have to be stopped for admitting the massecuite and thenagain for discharging the sugar crystals, thereby resulting in a loss oftime. There have been attempts to design continuous mode centrifugalseparators, but they have not been commercially successful due todefective separation, often accompanied by sugar crystal breakage.

Crystals from the centrifugal separator are typically characterized by amoisture content in the range 0.5-0.7%, while the end product isrequired to have a moisture content less than about 0.2%. Thus thecrystals are sent to dryers. These dryers are rather large drum devicesin which the sugar crystals to be dried are introduced and cascadedwithin the drum as a stream of hot air is directed through the drum toevaporate the moisture. Unfortunately, the cascading action causesconsiderable crystal breakage and actually causes a fraction of thesugar crystals to be transformed into a powdered form. This presentsserious dangers of explosion and thus necessitates costly precautionaryprocedures.

Continuous separation systems are known. In particular, U.S. Pat. No.701,687 to Desaulles discloses a rotary screen device wherein a fixedinlet chute deposits massecuite on a rotating screen while a screw augerremoves sugar crystals from the screen after substantially an entirerotation. Removal of the molasses is effected by a vacuum tank rotatingalong with the screen that serves to draw the liquid through the screen.While this system is capable of operating in a continuous mode, itnevertheless subjects the sugar crystals to excessive mechanicalhandling with the result of crystal breakage. Additionally, it should benoted that the crystals still require further handling such as purging,drying, etc., and the attendant costs and dangers associated therewith.

An alternate approach to the continuous recovery of commercial sugarcrystals and molasses has been tried, and employs a filtering belt and acasing under vacuum. During operation, the massecuite is fedcontinuously on top of the belt and the molasses recovered into thecasing by the suction action of the vacuum. The remaining sugar crystalsare eventually scraped off and removed from the belt and sent to adryer. The molasses is transferred from the casing to a degassifyingchamber before being pumped from the system. Apparently, problems havebeen encountered in maintaining the required vacuum within the casingdue to cracks in the sugar layer arising from the inability to keep abelt truly flat and level.

Accordingly, there is presented a need for a continuous separationsystem of simple design and operation which avoids the problem ofcrystal breakage.

SUMMARY OF THE INVENTION

The present invention provides an improved method of separating solidsand liquid in a slurry in a manner which continuously purges, washes,dries, and removes solids with a minimum amount of mechanical handlingand consequential crystal breakage.

A separator within the continuous separation system of the presentinvention comprises a stationary casing, a horizontal rotary screenwithin the casing, a fixed scraper mounted in the casing above thescreen, an inlet conduit on one side of the scraper for depositingslurry on the screen to form a layer as the screen rotates, an outletconduit on the other side of the scraper for carrying away dried solids,and gas nozzles for directing a gas stream at the solids as theyencounter the scraper, entraining them, and directing them through theoutlet conduit. The screen divides the casing interior into respectiveupper and lower chambers that are sealed from one another except throughthe screen.

In operation, the gas stream carries the entrained solids to a separatereceiver where the solids are gravitationally separated from the gasstream and carried off by a suitable conveyor. The gas is thenrecirculated and enters the upper chamber of the separator at a slightlypositive pressure. The lower chamber is evacuated so that a portion ofthe gas in the upper chamber passes through the slurry layer to displaceliquid through the screen while further amounts of the gas causeadditional drying of the solids so that they are of a suitably lowliquid content when they encounter the fixed scraper and are entrainedout of the separator. The recovered liquid is automatically degassifiedas it falls through the lower chamber prior to its removal.

It can immediately be appreciated that the use of a rigid screenassembly rotating at low speeds, and employing vacuum rather thancentrifugal force to effect liquid separation require less power,sophisticated equipment, and labor. Throughput is easily adjusted byeither varying the amount of material deposited on the screen, varyingthe speed of rotation of the screen, or both. The use of gas to entrainthe dried solids minimizes mechanical handling and consequentialbreakage, while using the gas to dry the solids minimizes the additionalmechanical handling the solids have to undergo.

According to a further aspect of the present invention that isespecially useful when the slurry is an aqueous slurry such asmassecuite, the gas that is first used to entrain the dried solids andthen recirculated to dry the slurry layer on the screen comprises drysteam under pressure. The steam becomes superheated as it expands uponentering the upper chamber. The use of steam provides numeroussurprising advantages. In particular, since the slurry is typicallycooler than the steam, the steam that contacts the slurry layer at aposition where it has recently been deposited momentarily condenses andthus dilutes the liquid and facilitates the separation or purgingprocess. However, as the water passes through the slurry layer, andencounters a region of lower pressure, the condensed water reevaporatesso that it does not add materially to the amount of wash water in therecovered liquid. At the same time, the steam that passes through thealready heated portions of the slurry layer does not condense, butrather becomes superheated as it passes through the layer, therebyrendering the drying effect more efficient. Thus, the steam is firstused for transporting the solids, and is then recirculated for purging,washing, and drying the slurry layer prior to removal of the solids bynewly introduced steam.

The use of steam is also advantageous since a high vacuum in the lowerchamber of the separator may be provided with a relatively small airpump. In particular, the system for evacuating the lower chamberincludes a condenser for condensing the steam after it passes out of thelower chamber. The improved vacuum facilitates and makes more efficientthe separation process.

For a more complete understanding of the nature and features of thepresent invention, reference should be had to the remaining portions ofthe specification taken with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified partially cut-away isometric schematic of theseparation system according to the present invention;

FIG. 2 is a simplified cut-away isometric view of the separator;

FIG. 3 is a cross-sectional view of the separator taken along line 3--3of FIG. 2; and

FIG. 4 is a cross-sectional view of the separator taken along line 4--4of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a simplified isometric schematic of a continuous separationsystem for separating solids and liquids contained in a body of slurry10 within a slurry vat 12. The separation system includes a separator15, a receiver 17, and an evacuation system 20. Separator 15 issupported by a frame 21 and comprises a casing 22 having respectiveupper and lower chambers 23 and 24. In operation, slurry in vat 12 isadmitted to upper chamber 23 of separator 15 through a slurry supplypipe 25 while a source of gas is admitted to upper chamber 23 through agas supply pipe 27. Evacuation system 20 communicates with lower chamber24 via a vacuum pipe 28. The operation of separator 15, as will bedescribed below, culminates in a separation of the solids and liquidwithin slurry 10. The liquid is drawn off from lower chamber 24 througha suction pipe 30 by a suitable pump 32. A valve 35 in suction pipe 30and a level sensor (not shown) cooperate to ensure that pump 32 isprimed at all times.

The separation and drying of the solids occur by means of the gassupplied through gas inlet 27. As will be described in greater detailbelow, the gas is first used to entrain the dried solids and carry themout of upper chamber 23 of separator 15 through a solids outlet conduit40 to receiver 17. The solids are discharged at the bottom of receiver17 onto a suitable conveyor 42 and transported elsewhere. The gas thatcarried the solids into receiver 17 is communicated back into upperchamber 23 through a recirculation conduit 45 for separating and dryingsubsequent amounts of slurry introduced into separator 15.

Separation is effected by a cooperation of the pressurized gasintroduced into upper chamber 23 through recirculation conduit 45 andthe vacuum maintained in lower chamber 24 through vacuum conduit 28.According to a particular aspect of the present invention, the gasintroduced through gas supply line 27 comprises a condensible vapor suchas steam, and evacuation system 20 comprises a barometric condensor 47and a vacuum pump 50. The operation of evacuation system 20 will bedescribed in detail below.

FIG. 2 is a simplified cut-away isometric view of separator 15. Ahorizontal rotary screen assembly 60 is mounted for rotation withincasing 22 and separates the interior of casing 22 into upper chamber 23and lower chamber 24. A stationary scraper assembly 62 is mounted abovescreen assembly 60. Screen assembly 60 has an annular liquid perviousportion extending over a radial range 70, and upper chamber 23 is ofgenerally annular configuration covering the entire radial extent ofrange 70. Lower chamber 24 is defined by a downwardly tapering lowerconical shell 72 connected to an upper cylindrical shell 75. Vacuum pipe28 communicates with lower chamber 24 through a port in cylindricalshell 75.

FIG. 3 is a cross-sectional view taken through line 3--3 of FIG. 2,showing in greater detail the construction of screen assembly 60 and themeans for effecting a seal between upper chamber 23 and lower chamber 24so that fluid communication therebetween only occurs through screenassembly 60. Screen assembly 60 includes a circular disc 80 having acentral hub 82 through which is concentrically attached the bottom end85 of a vertical shaft 87. Shaft 87 is journaled to frame 21 byappropriate bearings, and is driven by a motor 88 which is also attachedto frame 21. Disc 80 is preferably supported at its outer ends by aplurality of wheels 90 that ride on cooperating rails 92 affixed to theinner surface of cylindrical shell 75.

Disc 80 has on its upper surface a plurality of concentric grooves 95,the grooves extending over radial range 70. At the bottom of each groove95 is a plurality of circumferentially spaced apertures 97 that extendfrom the bottom of the groove to the lower surface of disc 80. Radialrange 70 is such that the innermost and outermost grooves are spacedfrom the center and the circumference, respectively, of disc 80. Anannular screen 100 overlies disc 80 and extends over the entire liquidpervious portion thereof, and extends radially inwardly and outwardlytherefrom by a small distance. Screen 100 is securely held flat by aninner circular retaining flange 102 and an outer circular retainingflange 105, retaining flanges 102 and 105 having respective upperinclined surfaces 107 and 110 that slope upwardly away from the surfaceof screen 100 in radially inward and outward directions, respectively.

Annular upper chamber 23 is defined by vertical inner and outercylindrical shells 112 and 115, and a horizontal annular plate 117. Thelower ends of cylindrical shells 112 and 115 are inclined to match withsmall clearance sloping surfaces 107 and 110 of retaining flanges 102and 105 holding screen 100 to disc 80. Cylindrical shell 112 carries aninwardly extending horizontal sealing flange 120; outer cylindricalshell 115 carries an outwardly extending horizontal sealing flange 122.Inner sealing flange 120 is provided with a downwardly facingcircumferential groove 125 in which a vertically adjustable rubber sealbearing 127 is disposed. Seal 127 bears against disc 80 at a pointradially inward of inner retaining flange 102. Similarly, outer sealingflange 122 has a circumferential groove 130 in which is located avertically adjustable rubber seal 132 that bears against disc 80 at apoint radially outward of outer retaining flange 105. Lower chambercylindrical shell 75 has an inwardly extending flange 135 that isfastened to outer sealing flange 122 in a gas-tight manner. Thus, upperchamber 23 is sealed from lower chamber 24 and communicates therewithonly through apertures 97 in disc 80 and screen 100.

The mechanism for introducing slurry into upper chamber 23 and removingdried solids is best seen with reference to FIGS. 2 and 4. FIG. 4 is across-sectional view taken along line 4--4 of FIG. 2. Scraper assembly62 comprises a partition plate 140 extending the entire radial andvertical dimensions of annular upper chamber 23, and a shoe-like scraper142 along its bottom edge and contacting screen 100. Scraper 142preferably has its bottom surface provided with a radially extendingrecess 143, at the top of which are disposed a number of holes 144communicating to a source of water (not shown) for continuously cleaningscreen 100 during operation. Additional vertical partition walls 145,147, and 150 further subdivide upper chamber 23 into additional smallerchambers. In particular, vertical partition plate 145 is angularlyspaced in a first direction from scraper partition plate 140 to definetherewith a slurry inlet chamber 152. Partition plate 147 is spaced in asecond opposite direction from scraper partition plate 140 to define aplenum chamber 155; partition plate 150 is further spaced in the seconddirection from partition plate 147 to define a solids outlet chamber157. A slanted plate 156 provides a gradual transition between chamber157 and solids outlet conduit 40. The remaining, major portion of upperchamber 22, designated by reference numeral 158, is the drying chamber.Partition plates 145 and 150 extend downwardly from plate 117 by adistance less than the entire vertical dimension of upper chamber 23.Partition plates 145 and 150 carry respective vertically movableextensions 159 and 160. The distance of these movable extensions fromscreen 100 is made adjustable from outside upper chamber 23 by suitablerack and pinion mechanisms 161 and 162. Partition plate 147 extendsdownwardly to within a small distance of scraper 142 to define ahorizontally extending narrow slot 165 communicating from plenum chamber155 to solids outlet chamber 157.

Slurry inlet conduit 25 communicates through a valve 167 to the top ofslurry inlet chamber 152. Gas line 27 communicates to plenum chamber 155through a valve 170, and further communicates downwardly into solidsoutlet chamber 157 through a valve 171, terminating in a horizontal,radially outwardly directed nozzle 172.

An array of spray nozzles 175 is disposed within drying chamber 158 at aposition generally proximate partition plate 145. Array of nozzles 175is typically provided by a plurality of radially extending manifoldtubes connected to an outside water source (not shown). These nozzlesare sometimes needed to wash the crystals.

Within the remaining portion of drying chamber 158 is an array ofstirrers in the form of downwardly extending vertical spikes 177extending to within a short distance from the top of screen 100. Thesestirrers enhance the drying process and prevent caking of the slurrylayer.

Referring again to FIG. 1, receiver 17 is in the form of a vessel 184having a conical bottom section 185 terminated by a solids dischargepipe 187 fitted with a valve 190. Solids outlet conduit 40 is horizontaland communicates with the chamber of receiver at an upper portionthereof. A central partition plate 192 extends downwardly below thepoint at which solids outlet conduit 40 enters receiver vessel 184.Plate 192 preferably has a resilient surface facing solids outletconduit 40 to minimize impact forces.

Barometric condenser 47 is in the form of a vessel 200 having adownwardly tapered bottom portion 202 which communicates to a verticalbarometric pipe 205 having its bottom end immersed in a pond of water207. Within condenser vessel 200 are located staggered horizontalbaffles 210 above which is located a water supply pipe 212. Vacuum pump50 communicates with vessel 200 at its uppermost portion, via a pipe215. Vacuum pipe 28 communicates with vessel 200 at a position belowbaffles 210 and above barometric pipe 205.

Having described the structure of the present invention, the operationmay now be understood. For definiteness, it is helpful to suppose thatit is required to separate commercial sugar crystals from massecuite, ahighly viscous mixture of molasses and sugar crystals obtained fromboiling cane juice. In particular, the procedure occurs as follows:

With valves 190 and 35 closed, lower chamber 24 is put under vacuumthrough vacuum pipe 28, barometric condenser 47, and pipe 215. A watercolumn of a height corresponding to the vacuum (preferably around 28inches mercury) within condenser 47 is established in barometric pipe205. Water is then introduced into vessel 200 through water intake 212,and cascades down baffles 210, into pond 207 via barometric pipe 205. Anoverflow canal is preferably provided for pond 207.

Dry steam under pressure and at a convenient temperature (e.g. 220° F.)is admitted into upper chamber 23 through horizontally extending slot165 and nozzle 172. The steam rushes into condenser 47 where it iscondensed into water which flows down through barometric pipe 205, alongwith the incoming cooling water. Any incondensable gases such as air,etc. entrained with the steam or the condenser cooling water arewithdrawn by vacuum pump 50.

Massecuite 10 is then allowed to flow from vat 12 at the correcttemperature (approximately 158° F.) into slurry inlet chamber 152 andseparator upper chamber 23, depositing itself on screen 100. As soon asthe massecuite within slurry inlet chamber 152 reaches a levelapproximately the entire height of upper chamber 23 motor 88 is startedso that the screen rotates in a direction such that the massecuiteimmediately on the screen begins moving away from scraper partitionplate 140 and into drying chamber 158. Movable extension 159 is adjustedso that its bottom is at a desired height (for example 4") above screen100 so that a massecuite layer 220 is formed and beings moving slowly inthe direction of arrow 222 as screen assembly 60 rotates. In themeantime since lower chamber 24 is under vacuum, separation commenceswith liquid molasses being sucked through the pervious portion of screenassembly 60 while a layer 225 of sugar crystals remains on top of screen100. As the sugar crystals continue their travel within drying chamber158, they are dried by the steam, as will be described below, ultimatelyencountering scraper 142. They are then scraped off screen 100, beingblown into receiver 115 by the combined actions of the steam jets fromslot 165 and nozzle 172. The steam flowing out of slot 165 blows thesugar crystals generally upwardly, partially fluidizing them, so thatnozzle 172 may blow them out through solids outlet conduit 40.

The level of massecuite layer 220 is higher than the level of sugarcrystal layer 225 due to the removal of molasses during the separationprocess. Movable extension 160 is adjusted to provide a small clearanceabove the top of layer 225 of sugar crystals in order to prevent theincoming steam within solids outlet chamber 157 from expanding intodrying chamber 158. Thus the steam expands through conduit 40 tofacilitate the expulsion of the sugar crystals.

Once the sugar crystals have been entrained in the flow of steam, andenter receiver 17, the steam and entrained sugar crystals encountersvertical partition plate 912 and are deviated downwardly toward thebottom of receiver 17. The sugar crystals, assisted by gravity, aredeposited while the steam continues upwardly around the bottom ofpartition plate 192 and then through recirculation conduit 45 intodrying chamber 158.

This process continues until the sugar crystals in receiver 17 reach aconvenient height, at which time valve 190 is opened and conveyer 42 isstarted. The sugar crystals drop through solids discharge pipe 187 ontothe conveyer to direct them to a sugar storage area or the like. Thesugar crystals must be kept at a convenient height within receiver 17 inorder to seal the interior of chamber 184 from the atmosphere.

Upon reaching drying chamber 158, the expanded steam, whose initialpressure, temperature, and volume is such to maintain a slight positivepressure within drying chamber 158, is slighly superheated and occupiesthe entire volume within chamber 158 on top of the material beingprocessed.

Thus, the steam above the layer of material (slurry when it enters andcrystals when it leaves) is at a slightly positive pressure while theregion underneath screen assembly 60 is under vacuum. Accordingly, thesteam is forced through the material being processed, entraining with itthe molasses contained in the massecuite during the early stages of theprocess (that is for the material proximate slurry inlet chamber 152),and entraining the moisture contained in the layer of sugar solidsduring the later stages of the process (as they approach solids outletchamber 157).

During the early stages, the material is at a much lower temperaturethan that of the steam (approximately 158° F. as opposed to 220° F.),thus causing part of the steam to momentarily condense on top of theslurry. In addition to diluting the molasses the condensation of thesteam heats the molasses, thus lowering its viscosity to furtherfacilitate the separation or purging process which usually takes placein 5 to 10 seconds, depending on the thickness of the slurry involved.However, due to the high pressure difference existing between upperchamber 23 and lower chamber 24, the condensed water startsreevaporating as it approaches screen 100 so that it subsequently leaveslower chamber 24 in a vapor state. This further offers the advantage ofminimizing the amount of wash water in the recovered molasses. Thepurpose of spray nozzles 175 is to provide for further washing if itturns out that the effect of the temporarily condensed steam is notsufficient for proper purging.

During the later stages of the process, the sugar crystals have beenbrought up to a temperature almost equal to that of the steam, so thatcondensation does not occur. Therefore, the steam has the effect ofdrying the sugar crystals, this drying effect being more efficient asthe steam progressively expands and becomes superheated on its waythrough the sugar crystals. Stirrers 177 serve the purpose of plowingthe layer of sugar crystals in order to further improve the efficiencyof the drying process. In order to minimize heat losses, receiver 17 andupper chamber 23 are preferably insulated on the outside.

The liquid molasses that collects in lower chamber 24 is automaticallydegassified by the action of the vacuum before being withdrawn throughsuction pipe 30 to appropriate storage tanks. The liquid molassestypically boils under the vacuum, thereby causing a portion of the waterin the molasses to vaporize and pass to condenser 47 in a vapor state.

It is possible to use two or more separation systems in conjunction witha single common condenser, in which case vacuum conduit 28 wouldcommunicate with a vacuum manifold, the vacuum manifold being incommunication with condenser 47.

As a general matter, the higher the vacuum within lower chamber 24, thebetter the separation effect. The use of steam in connection with acondenser provides an excellent method of maintaining an extremely highvacuum while requiring a relatively small vacuum pump. The reason thatthis is so is that during operation, the steam never reaches vacuum pump50, but rather is condensed in condenser 47 and removed throughbarometric pipe 205. Naturally, the present system may be used with hotair, but in that case a vacuum pump capable of sustaining the requiredvacuum without the benefit of the condensation phenomenon is required.

In summary, it can be seen that the present invention provides acontinuous separation system which removes dried solids in a gas streamwith a minimum amount of mechanical handling, and provides the solids ata sufficiently low moisture content that subsequent drying operationsare not required. Additionally, the use of steam to continuously purge,wash, dry and remove solids allows the process to be carried outeconomically, simply, and effectively.

While the above provides a full and complete disclosure of the preferedembodiments of the invention, various modifications, alternateconstructions, and equivalents may be employed without departing fromthe true spirit and scope of the invention. For example, while arecirculating system wherein the gas is used first for entraining thedried solids and subsequently for drying additional solids isadvantageous, there may be some circumstances where separate gas sourcesmight be desirable. In particular, it might be useful to entrain thesolids with air and to dry them with steam. Therefore, the abovedescription and illustrations should not be construed as limiting thescope of the invention which is defined by the appended claims.

I claim:
 1. A method of separating solid and liquid material in a slurrylayer on a screen comprising the steps of:continuously depositing saidslurry on a moving liquid pervious member to form a slurry layerthereon; introducing a condensible vapor at a first surface of saidlayer; reducing the pressure at a second surface of said layer withrespect to the pressure at said first surface in order to cause liquidto flow through said liquid pervious member so that the liquid contentof said slurry layer becomes progressively lower as said layer iscarried on said moving liquid pervious member; withdrawing liquid fromsaid second surface of said layer, said vapor passing through said layerand becoming superheated upon such passage therethrough, saidsuperheated vapor evaporating further liquid from said solids to drythem; directing a stream of gas at said slurry layer at a point where itconsists essentially of solids to entrain said solids and remove themfrom said liquid pervious member; and separating said entrained solidsfrom said gas stream.
 2. The invention of claim 1 wherein an initialamount of said vapor initially condenses on said layer to purge and washsaid solids of said liquid.
 3. The invention of claim 1 wherein saiddirecting step comprises the step of directing a stream of the samecondensible vapor as is introduced in said introducing step.
 4. Theinvention of claim 3, and further comprising the step of directing saidstream of condensible vapor after separation of said solids therefrom tosaid slurry layer on said liquid pervious member at the upper surfacethereof to at least partially define the condensible vapor that isintroduced in said introducing step.
 5. A method of separating solid andliquid material in a slurry comprising the steps of:continuouslydepositing said slurry on a moving liquid pervious member to form aslurry layer thereon; maintaining a pressure differential across saidslurry layer with the pressure above said slurry layer being in excessof the pressure beneath said slurry layer in order to cause liquid toflow through said liquid pervious member so that the liquid content ofsaid slurry layer becomes progressively lower as said layer is carriedon said moving liquid pervious member; directing a stream of gas at saidslurry layer at a point where it consists essentially of solids toentrain said solids and remove them from said liquid pervious member;and separating said entrained solids from said gas stream; directingsaid gas stream after separation of said solids therefrom to said slurrylayer on said liquid pervious member at the upper surface thereof to atleast partially contribute to said pressure differential, and to purgesaid slurry of liquid and dry remaining solids that have already beenpurged of liquid.
 6. The invention of claim 5 wherein said gas is acondensible vapor of a liquid that is miscible with the liquid in saidslurry and wherein said gas stream introduced above said slurry layer isat a temperature above the temperature at which said slurry is depositedon said liquid pervious member, such that a portion of said vaporinitially condenses on at least a portion of said layer to dilute andheat the liquid in said slurry and aid in said purging, and elevates thetemperature of said solids, and a further portion of said vapor whenpassing through said layer becomes superheated on such passage to causeevaporation of liquid from said solids.
 7. A method of separating solidand liquid material in a slurry layer on a screen comprising the stepsof:forming a slurry layer; transporting said slurry layer from an inletlocation to an outlet location; maintaining a pressure differentialacross said slurry layer with the pressure above said slurry layer beingin excess of the pressure beneath said slurry layer in order to causeliquid to flow through said slurry layer so that the liquid content ofsaid slurry layer becomes progressively lower as said layer moves towardsaid outlet location; directing a stream of gas at said slurry layer ata point proximate said outlet location to entrain said solids; andseparating said entrained solids from said gas stream.
 8. The inventionof claim 7, and further comprising the step of directing said gas streamafter separation of said solids therefrom to a region above said slurrylayer to at least partially contribute to said pressure differential,and to purge said slurry of liquid and dry remaining solids that havealready been purged of liquid.
 9. The invention of claim 7 wherein saidmaintaining step includes the step of introducing above said slurrylayer a condensible vapor of a liquid that is miscible with the liquidin said slurry and wherein said vapor introduced above said slurry layeris at a temperature above the temperature of said slurry layer near saidinlet location, such that a portion of said vapor initially condenses onat least a portion of said layer to dilute and heat the liquid in saidslurry and aid in said purging, and elevates the temperature of saidsolids, and a further portion of said vapor when passing through saidlayer becomes superheated to cause evaporation of liquid from saidsolids.
 10. The invention of claim 1 or 6 or 7 wherein said step ofintroducing a condensible vapor comprises the step of introducing steam.11. The invention of claim 1 or 6 or 7 wherein the step of reducing thepressure comprises the substeps of:withdrawing said vapor and liquidevaporated by the passage of said vapor through said layer from saidsecond surface; and condensing said vapor and evaporated liquid tocreate and maintain a vacuum at said second surface.